I used bells because before we had some percussive sounds but they were in the same frequency range as the gunfire and explosions and so they couldn't be heard. Now I use the bells and they cut through the gunfire, so I'd like that any replacements also cut through the gunfire in the same way. It's pretty important for the design that players attention is brought to the bar filling up, so those sounds need to come through loud and clear. If someone sends me sounds but I can't hear them as much as I can hear the bells now, I probably won't merge them into the game. That said I think the current sounds could use a lot of improvement.